The objectives of the proposed investigation of Chinese people living in areas of high background radiation are to learn whether continuous exposure to such radiation may be related to health effects, especially damage to the thyroid gland. The objectives would be accomplished by physical examination of 2,000 women who are over age 50 in both the high background and control areas. Physical examination will be performed by three expert thyroidologists. Blood samples would be taken on a one in 10 sample for chromosome analyses and on all women with palpable thyroid abnormalities for further diagnostic testing. Laboratory tests to determine thyroxine (T4), thyrotropin (TSH), and thyroid microsomal antibodies will assist in the evaluation of thyroid disease. Needle biopsies will be performed on all persons with palpable nodules. Chromosome evaluations would be made to investigate any differences in stable aberrations between the two study groups. The proposed study has broader implications than evaluating hazards from background radiation and may provide useful guidance in risk assessment for other populations exposed to frequent, but low, doses of ionizing radiation, whether for environmental, medical or occupational reasons.